Method for producing zeta negative nanodiamond dispersion and zeta negative nanodiamond dispersion

ABSTRACT

The present invention relates to a method for producing zeta negative single digit carboxylated nanodiamond dispersion. The method comprises adjusting pH of zeta negative carboxylated nanodiamond suspension to at least 7, and subjecting the pH adjusted suspension to beads milling. The present invention further relates to zeta negative single digit carboxylated nanodiamond dispersion comprising zeta negative single digit carboxylated nanodiamond particles and a liquid medium, wherein zeta potential of the zeta negative single digit carboxylated nanodiamond dispersion is over −37 mV measured at pH over 7, zeta negative single digit carboxylated nanodiamond particle concentration in the dispersion is over 2 wt-% and D90 average primary particle size distribution of the zeta negative single digit carboxylated nanodiamond particles is from 2 nm to 12 nm.

PRIORITY

This application is a continuation application of U.S. Ser. No.14/785,694, which is a US national application of internationalapplication PCT/FI2014/050290, filed on Apr. 22, 2014, and claimspriority of Finnish national application number FI20135416 filed on Apr.23, 2013, contents of all of which are incorporated herein by reference.

FIELD OF THE INVENTION

The present invention relates to a method for producing zeta negativesingle digit carboxylated nanodiamond dispersion and to zeta negativesingle digit carboxylated nanodiamond dispersion.

BACKGROUND ART

Nanodiamond (ND) also referred to as ultrananocrystalline diamond orultradispersed diamond (UDD) is a unique nanomaterial which can easilybe produced in hundreds of kilograms by detonation synthesis.

Detonation nanodiamonds (ND) were first synthesized by researchers fromthe USSR in 1963 by explosive decomposition of high-explosive mixtureswith negative oxygen balance in a non-oxidizing medium. A typicalexplosive mixture is a mixture of trinitrotoluene (TNT) and hexogen(RDX) and a preferred weight ratio of TNT/RDX is 40/60.

As a result of the detonation synthesis a diamond-bearing soot alsoreferred to as detonation blend is obtained. This blend comprisesnanodiamond particles, which typically have an average particle size ofabout 2 to 8 nm, and different kinds of non-diamond carbon contaminatedby metals and metal oxide particles coming from the material of thedetonation chamber. The content of nanodiamonds in the detonation blendis typically between 30 and 75% by weight.

The nanodiamond-containing blends obtained from the detonation containsame hard agglomerates, typically having a diameter of above 1 mm. Suchagglomerates are difficult to break. Additionally the particle sizedistribution of the blend is very broad.

The diamond carbon comprises spa carbon and the non-diamond carbonmainly comprises sp² carbon species, for example carbon onion, carbonfullerene shell, amorphous carbon, graphitic carbon or any combinationthereof.

There are number of processes for the purification of the detonationblends. The purification stage is considered to be the most complicatedand expensive stage in the production of nanodiamonds.

For isolating the end diamond-bearing product, a complex of chemicaloperations directed at either dissolving or gasifying the impuritiespresent in the material are used. The impurities, as a rule, are of twokinds: non-carbon (metal oxides, salts etc.) and nondiamond forms ofcarbon (graphite, black, amorphous carbon).

Chemical purification techniques are based on the different stability ofthe diamond and non-diamond forms of carbon to oxidants. Liquid-phaseoxidants offer an advantage over gas or solid systems, because theyallow one to obtain higher reactant concentrations in the reaction zoneand, therefore, provide high reaction rates.

In the recent years nanodiamonds have received more and more attentiondue to several existing applications within electroplating (bothelectrolytic and electroless), polishing, various polymer mechanical andthermal composites, CVD-seeding, oils and lubricants additives as wellas possible new applications such as luminescence imaging, drugdelivery, quantum engineering etc.

The fact that the available nanodiamond materials possess a variety ofvarious surface functions and thus agglomeration (from several hundredsof nanometers to several microns), is effectively limiting their use inindustrial applications. As applying agglomerating nanodiamond grades,very high filler loadings are typically required, making their costefficient use impossible in most of the applications today. Moreover,nanodiamond agglomeration is effectively limiting or prohibiting theoptimization of various application end product technical properties.Agglomeration is making it impossible to use nanodiamonds inapplications wherein the product optical properties have to be retained;agglomeration is causing scratching in polishing and fine-polishingapplications; agglomeration can have a direct adverse effect on polymercomposite mechanical properties; agglomeration in an electroplatingelectrolyte or electroless deposition chemicals (due to non-optimalnanodiamond zeta potential as in relation to electrolyte pH regime)makes their usage completely impossible for manufacturing mechanicallyimproved metal coatings; agglomeration is effectively prohibitingnanodiamond usage as a drug carrier material; agglomeration is having anadverse effect on CVD produced diamond film quality etc.

Cost efficient and technologically optimized usage of nanodiamondmaterials both in their powder, suspension and dispersion form can onlybe achieved if nanodiamonds are substantially mono-functionalized typeand have thus, depending on the type of surface modification, thehighest possible affinity to various solvents and polymer, metal orceramic materials. This substantial mono-functionalization should alsoaccomplish the production of single digit nanodiamond dispersions(dispersions wherein nanodiamonds are essentially in their primaryparticle, agglomerate free form). Such a substantiallymono-functionalized nanodiamond possesses, depending on the type ofsurface functionalization, either a highly positive or negative zetapotential value.

The significance of zeta potential is that its value can be related tothe stability of colloidal dispersions. The zeta potential indicates thedegree of repulsion between adjacent, similarly charged particles indispersion. For molecules and particles that are small enough, a highzeta potential will confer stability, i.e., the solution or dispersionwill resist aggregation. When the potential is low, attraction exceedsrepulsion and the dispersion will break and flocculate. So, colloidswith high zeta potential (negative or positive) are electricallystabilized while colloids with low zeta potentials tend to coagulate orflocculate. If the zeta potential is 0 to ±5 mV, the colloid issubjected to rapid coagulation or flocculation. Zeta potential valuesranging from ±10 mV to ±30 mV indicate incipient instability of thecolloid (dispersion), values ranging from ±30 mV to ±40 mV indicatemoderate stability, values ranging from ±40 mV to ±60 mV good stabilityas excellent stability is reached only with zeta potentials more than±60 mV.

In order to be technologically viable and cost efficient, nanodiamondsingle digit dispersions should possess high nanodiamond concentrations.Preferably, nanodiamond concentrations should exceed 2 wt-%. If theconcentration is too low, too much of excess solvent has to be addedinto application processes, having an adverse effect on metal ion andother additive concentration within electroplating, polymer resincontent in paints and lacquers changing thus drastically the curingprocess etc.

Several methods for functionalizing the nanodiamonds with differentfunctional groups have been developed. Typical functionalizednanodiamonds are carboxylated nanodiamonds, hydroxylated nanodiamondsand hydrogenated nanodiamonds, but contain still a mixture of typicallyoppositely charged functions and thus, mediocre zeta potential valuesand are thus not available in their solvent dispersion forms.

In prior art, methods, such as beads milling, to produce highly zetanegative, single digit carboxylated nanodiamond dispersions have beenproposed. However, the methods have not been very successful.

It is known in the art that when subjecting a highly zeta negative,carboxylated nanodiamond powder to beads milling, a big part of existingsurface carboxylic acid functions are subjected to reduction to zetapositive hydroxyl groups. This reduction is having an adverse effect onthe material overall zeta potential, and results in severeagglomeration. Eventually the agglomerates clog the beads millingapparatus.

Publication A. Krueger and D. Lang, Adv. Funct. Mater. 2012, 22, 890-906discloses the phenomena where nanodiamond particles, subjected to abeads assisted sonic disintegration (a method combining beads millingand ultrasonication), become significantly more hydrophilic, zetapotential rises to ˜+40 mV (at neural pH) and colloidal solutions of OHterminated nanodiamond particles are obtained. Based on abovedisclosure, there is a qualitative and quantitative need for anefficient method for producing highly zeta negative single digitcarboxylated nanodiamond dispersion and a product thereof.

SUMMARY OF THE INVENTION

The present invention relates to a method for producing zeta negativesingle digit carboxylated nanodiamond dispersion.

The present invention further relates to zeta negative single digitcarboxylated nanodiamond dispersion.

It has now been surprisingly found that by adjusting pH of zeta negativecarboxylated nanodiamond suspension into neutral to alkaline region bybase additives it is possible to block nanodiamond carboxylic acidfunctions from chemo-mechanical reduction to respective hydroxyl groupsduring beads milling process. As no reduction takes place, the millingcan now be carried out without any nanodiamond agglomeration and thus,also clogging is avoided too. As the nanodiamond carboxylic acidfunctions are protected from said reduction, highly zeta negative singledigit carboxylated nanodiamond dispersions in high concentrations can beproduced by the beads milling process.

BRIEF DESCRIPTION OF FIGURES

FIG. 1 shows zeta potential of carboxylated nanodiamonds during beadsmilling according to a reference method.

FIG. 2 shows particle size of carboxylated nanodiamonds during beadsmilling according to a reference method.

FIG. 3 shows nanodiamond precipitation in a hose of a beads millapparatus in a reference method.

FIG. 4 shows clogging of a sieve system of a beads mill apparatus in areference method.

FIG. 5 shows zeta potential of carboxylated nanodiamond dispersion afterbeads milling according to the present invention.

FIG. 6 shows particle size distribution of carboxylated nanodiamonddispersion after beads milling according to the present invention.

FIG. 7 shows dispersion stability of pH adjusted carboxylatednanodiamond dispersion samples within pH range of 2 to 13 according topresent invention.

DETAILED DESCRIPTION

By term “zeta negative nanodiamond” is meant a nanodiamond particlehaving a negative zeta potential.

By term “carboxylated nanodiamond” is meant a nanodiamond particlehaving carboxylic acid functions on its surface.

By term “zeta negative carboxylated nanodiamond” is meant a nanodiamondparticle having carboxylic acid functions on its surface and having anegative zeta potential.

By term “zeta negative single digit carboxylated nanodiamond” is meant ananodiamond particle substantially in its primary particle form havingcarboxylic acid functions on its surface and having a negative zetapotential.

By term “zeta negative single digit carboxylated nanodiamond dispersion”is meant a dispersion of a liquid medium and nanodiamond particles wherethe nanodiamond particles are substantially present in their primaryparticle form and having carboxylic acid functions on theirs surface,and the dispersion having a negative zeta potential.

In a first aspect of the present invention there is provided a methodfor producing zeta negative single digit carboxylated nanodiamonddispersion.

More particularly there is provided a method for producing zeta negativesingle digit carboxylated nanodiamond dispersion, the method comprisingadjusting pH of zeta negative carboxylated nanodiamond suspension to atleast 7, and subjecting the pH adjusted suspension to beads milling.

The zeta negative carboxylated nanodiamond suspension comprises zetanegative carboxylated nanodiamond particles and a liquid medium.

The zeta negative carboxylated nanodiamond particles may be essentiallypure nanodiamond particles, preferably having a nanodiamond content ofat least 87% by weight, more preferably at least 97% by weight. Thecarboxylated nanodiamond particles may contain graphite and amorphouscarbon originating from the production of the nanodiamonds. They mayalso contain some residual metal impurities, either as metals or inmetal oxide form.

The zeta negative carboxylated nanodiamond particles may be in thesuspension in agglomerated form, or as a mixture of agglomerated andsingle digit form. In one embodiment, the zeta negative carboxylatednanodiamond particles are in single digit form in the liquid mediumbefore the beads milling.

Particle size distribution of the agglomerated suspension form isbetween 2 nm and 400 nm, preferably between 2 nm and 100 nm.

The zeta negative carboxylated nanodiamond particles are commerciallyavailable.

The liquid medium of the suspension may be any suitable liquid medium.The liquid medium is preferably selected from the group consisting ofpolar protic solvents, polar aprotic solvents, dipolar aprotic solvents,ionic liquids, or a mixture of any said mediums.

Preferred polar protic solvents are water; alcohols such as methanol,ethanol, iso-propanol, butanol, linear aliphatic diols such as ethyleneglycol, 1,3-propanediol, 1,4-butanediol, 1,5-pentanediol,1,8-octanediol; branched diols such as 1,2-propanediol, 1,3-butanediol,2,3-butanediol, 1,3-butanediol, 1,2-pentanediol, etohexadiol,p-menthane-3,8-diol, 2-methyl-2,4-pentanediol; and carboxylic acids suchas formic acid and acetic acid.

Preferred polar aprotic solvents are dichloromethane, tetrahydrofuran,propylene carbonate and lactams such as N-methyl-2-pyrrolidone (NMP) andN-ethyl-2-pyrrolidone (NEP).

Preferred dipolar aprotic solvents are ketones such as acetone andmethyl ethyl ketone (MEK); esters such as methyl acetate, ethyl acetate;N,N-methylformamide and dimethyl sulfoxide (DMSO).

Preferred ionic liquids are 1-ethyl-3-methylmidazolium chloride,1-butyl-3-methylmidazolium chloride, 1-ethyl-3-methyl-imidazoliumethylsulfate, 1-ethyl-3-methyl imidazolium diethylphosphate,1-ethyl-3-methyl-imidazolium dicyanamide,Tris-(2-hydroxyethyl)-methylammonium methylsulfate,1-ethyl-3-methyl-imidazolium thiocyanate, 1-ethyl-3-methyl-imidazoliumtetrafluoroborate, 1-ethyl-3-methyl-imidazoliumtrifluoromethanesulfonate, 1-ethyl-3-methyl-imidazolium bis(trifluoromethanesulfonyl)imide, 1-ethyl-3-methyl-imidazoliummethylcarbonate and 1-butyl-3-methyl-imidazolium methylcarbonate, Themost preferred ionic liquids are 1-ethyl-3-methylmidazolium chloride and1-butyl-3-methylmidazolium chloride.

More preferably the liquid medium is selected from the group consistingof water, methanol, ethanol, iso-propanol, linear aliphatic diols,branched diols, N-methyl-2-pyrrolidone (NMP), N-ethyl-2-pyrrolidone(NEP) and dimethyl sulfoxide (DMSO), or a mixture of any said solvents.

Most preferably the liquid medium is water.

pH of the zeta negative carboxylated nanodiamond suspension is typicallyat acidic region. That is, the pH is typically under 7.

In the present invention, the pH of the zeta negative carboxylatednanodiamond suspension is adjusted into neutral to basic region. Thatis, the pH of the suspension is adjusted to at least 7. Preferably thepH is adjusted to between 7 and 14, more preferably between 7 and 13.

The pH may be adjusted by any known method, e.g. by mixing the zetanegative carboxylated nanodiamond suspension and a base with ultrasonictreatment, with or without additional mechanical stirring.

The pH of the zeta negative carboxylated nanodiamond suspension may beadjusted with any suitable weak or strong base, such as Brönsted orLewis bases. Preferably the pH is adjusted with ammonium hydroxide,ammonia, NaH₄OH, methyl amine, diethyl amine, pyridine, tri-methylammonia, lithium hydroxide (LiOH), sodium hydroxide (NaOH), potassiumhydroxide (KOH), barium hydroxide (Ba(OH)₂), caesium hydroxide (CsOH),strontium hydroxide (Sr(OH)₂), calcium hydroxide (Ca(OH)₂), rubidiumhydroxide (RbOH), more preferably ammonium hydroxide, ammonia, NaH₄OH,NaOH, KOH, and most preferably with ammonium hydroxide.

The pH of the zeta negative carboxylated nanodiamond suspension may belowered if necessary with an acid, such as strong or weak acid, howeverin the way that the final pH of the suspension is at least 7.

Prior the pH adjustment, zeta potential of the zeta negativecarboxylated nanodiamond suspension is over −30 mV measured at pH from 3to 5, preferably over −35 mV at pH from 3 to 5, more preferably over −40mV at pH from 3 to 5.

In a preferred embodiment, before the beads milling of the pH adjustedzeta negative carboxylated nanodiamond suspension, the zeta negativecarboxylated nanodiamond suspension is subjected to a pre-treatment,preferably to a pre-treatment that lowers the intermolecularinteractions of the zeta negative carboxylated nanodiamonds. Preferablythe pre-treatment method is ultrasonication. Any typical ultrasonicdevice known by a skilled person may be used.

The zeta negative carboxylated nanodiamond suspension may be pre-treatedbefore the pH adjustment, after the pH adjustment, or during the pHadjustment. Preferably the zeta negative carboxylated nanodiamondsuspension is pre-treated with ultrasonication during the pH adjustment.

After the pH adjustment, and optional pre-treatment of the zeta negativecarboxylated nanodiamond suspension, the suspension is subjected tobeads milling.

Beads milling, or bead milling, is a commonly used term and known for askilled person.

A beads mill is a type of a mechanical grinder. It is a cylindricaldevice used in grinding (or mixing) various materials. The mills arefilled with the material to be ground and grinding media. Differentmaterials are used as grinding media, including ceramic balls, flintpebbles and stainless steel balls. An internal cascading effect reducesthe material to a fine powder. Beads mills can operate continuously orperiodically, and can grind various materials either wet or dry.

Beads mill apparatuses can be operated with pass or re-circulationmethod. In the pass method the material is fed to the mill at one endand discharged at the other end. In the re-circulation method thematerial to be grinded circulates in the system until required particlesize is obtained. The smaller the grinding media particles are, thesmaller is the particle size of the final product. At the same time, thegrinding media particles should be larger than the largest pieces of thematerial to be ground.

A grinding chamber of a beads mill can also be filled with an inertshield gas, such as nitrogen, that does not react with the materialbeing ground, to prevent oxidation or explosive reactions that couldoccur with ambient air inside the mill.

The beads for the beads mill are chosen so that the milled particleswill have a suitable diameter. In the present invention the beads millis run with beads having diameter preferably from 10 μm to 100 μm, morepreferably from 20 to 50, and most preferably 30 μm.

The beads milling is run until a suitable particle size is obtained. Inthe present invention obtained zeta negative carboxylated nanodiamondparticles in the dispersion are substantially in a single digit form.

In a preferred embodiment beads milling of the pH adjusted zeta negativecarboxylated nanodiamond suspension is assisted by ultrasonication. Thatis, the pH adjusted zeta negative carboxylated nanodiamond suspension issimultaneously beads milled and treated with ultrasonication. Combinedbeads milling and ultrasonication is also known as a beads assistedsonic disintergration process (BASD process). BASD apparatuses arecommercially available. The ultrasonication can be kept on during thewhole milling or switched off at any stage, and optionally switched onagain.

The zeta negative single digit carboxylated nanodiamond dispersionsobtained from the beads mill process have an average primary particlesize distribution (D90) from 2 nm to 30 nm, preferably from 2 nm to 20nm, more preferably 2 to 12 nm, most preferably from 3 nm to 8 nm.

Zeta potential of the obtained zeta negative single digit carboxylatednanodiamond dispersion is over −35 mV, preferably over −37 mV, morepreferably over −40 mV and even more preferably over −50 mV measured atpH higher than 7.

Zeta potential of the obtained zeta negative single digit carboxylatednanodiamond dispersion is preferably over −60 mV, and more preferablyover −70 mV, measured at pH higher than 8.5.

The zeta negative single digit carboxylated nanodiamond dispersion isstable when the zeta potential is over −37 mV measured at pH higher than7.

Concentration of the zeta negative single digit carboxylated nanodiamondparticles in the obtained dispersion is over 1 wt-%, preferably over 2wt-%, more preferably between 2 and 10 wt-%, most preferably between 3and 8 wt-%.

pH of the obtained zeta negative single digit carboxylated nanodiamonddispersion may be basic, neutral or acidic. Preferably the pH is from4.5 to 14, preferably from 5 to 13, more preferably from 7 to 13.

The obtained zeta negative single digit carboxylated nanodiamonddispersion may be further processed. pH of the dispersion may beadjusted. The pH may be adjusted, depending on the pH of the dispersion,to acidic region or to basic region or to neutral. Preferably the pH isadjusted between 8 and 12, more preferably between 9 and 12, mostpreferably between about 10 and about 11. The pH can be adjusted withany suitable weak or strong base or acid. The dispersion may also becentrifuged, filtered or nanodiamond concentration can be adjusted byevaporation or dilution.

With the method of the present invention beads milling of zeta negativecarboxylated nanodiamond suspension is performed without problems causedby clogging of the beads mill. By adjusting pH of zeta negativecarboxylated nanodiamond suspension into neutral to alkaline region bybase additives, nanodiamond carboxylic acid functions are blocked fromchemo-mechanical reduction to their respective hydroxyl groups duringbeads milling process. As no reduction takes place, the beads milling toproduce highly zeta negative nanodiamond single digit dispersions can becarried out successfully. As the nanodiamond carboxylic acid functionsare blocked from reduction, highly zeta negative, single digitnanodiamond dispersions in high concentrations can be produced by theprocess of the present invention.

In a second aspect of the present invention there is provided a zetanegative single digit carboxylated nanodiamond dispersion comprisingzeta negative single digit carboxylated nanodiamond particles and aliquid medium.

More particularly there is provided a zeta negative single digitcarboxylated nanodiamond dispersion comprising zeta negative singledigit carboxylated nanodiamond particles and a liquid medium, wherein

i) zeta potential of the zeta negative single digit carboxylatednanodiamond dispersion is over −37 mV measured at pH higher than 7,

ii) zeta negative single digit carboxylated nanodiamond particleconcentration is over 2 wt-% in the dispersion,

iii) D90 average primary particle size distribution of the zeta negativesingle digit carboxylated nanodiamond particles is from 2 nm to 12 nm.

The liquid medium of the dispersion may be any suitable liquid medium.The liquid medium is preferably selected from the group consisting ofpolar protic solvents, polar aprotic solvents, dipolar aprotic solvents,ionic liquids, or a mixture of any said mediums.

Preferred polar protic solvents are water; alcohols such as methanol,ethanol, iso-propanol, butanol, linear aliphatic diols such as ethyleneglycol, 1,3-propanediol, 1,4-butanediol, 1,5-pentanediol,1,8-octanediol; branched diols such as 1,2-propanediol, 1,3-butanediol,2,3-butanediol, 1,3-butanediol, 1,2-pentanediol, etohexadiol,p-menthane-3,8-diol, 2-methyl-2,4-pentanediol; and carboxylic acids suchas formic acid and acetic acid.

Preferred polar aprotic solvents are dichloromethane, tetrahydrofuran,propylene carbonate and lactams such as N-methyl-2-pyrrolidone (NMP) andN-ethyl-2-pyrrolidone (NEP).

Preferred dipolar aprotic solvents are ketones such as acetone andmethyl ethyl ketone (MEK); esters such as methyl acetate, ethyl acetate;N,N-methylformamide and dimethyl sulfoxide (DMSO).

Preferred ionic liquids are 1-ethyl-3-methylmidazolium chloride,1-butyl-3-methylmidazolium chloride, 1-ethyl-3-methyl-imidazoliumethylsulfate, 1-ethyl-3-methyl imidazolium diethylphosphate,1-ethyl-3-methyl-imidazolium dicyanamide,tris-(2-hydroxyethyl)-methylammonium methylsulfate,1-ethyl-3-methyl-imidazolium thiocyanate, 1-ethyl-3-methyl-imidazoliumtetrafluoroborate, 1-ethyl-3-methyl-imidazoliumtrifluoromethanesulfonate, 1-ethyl-3-methyl-imidazolium bis(trifluoromethanesulfonyl)imide, 1-ethyl-3-methyl-imidazoliummethylcarbonate and 1-butyl-3-methyl-imidazolium methylcarbonate, Themost preferred ionic liquids are 1-ethyl-3-methylmidazolium chloride and1-butyl-3-methylmidazolium chloride.

More preferably the liquid medium is selected from the group consistingof water, methanol, ethanol, iso-propanol, linear aliphatic diols,branched diols, N-methyl-2-pyrrolidone (NMP), N-ethyl-2-pyrrolidone(NEP) and dimethyl sulfoxide (DMSO), or a mixture of any said solvents.

Most preferably the liquid medium is water.

Zeta potential of the zeta negative single digit carboxylatednanodiamond dispersion is over −37 mV, more preferably over −40 mV andmost preferably over −50 mV measured at pH higher than 7.

Zeta potential of the zeta negative single digit carboxylatednanodiamond dispersion is preferably over −60 mV, and more preferablyover −70 mV, measured at pH higher than 8.5.

The zeta negative single digit carboxylated nanodiamond dispersion isstable when the zeta potential is over −37 mV measured at pH higher than7.

Concentration of the zeta negative single digit carboxylated nanodiamondparticles in the dispersion is over 2 wt-%, more preferably between 2and 10 wt-%, and most preferably between 3 and 8 wt-%.

D90 average primary particle size distribution of the zeta negativesingle digit carboxylated nanodiamond particles is preferably from 3 nmto 8 nm.

pH of the zeta negative single digit carboxylated nanodiamond dispersionmay be basic, neutral or acidic. Preferably the pH is from 4.5 to 14,preferably from 5 to 13, more preferably from 7 to 13.

In the following the invention will be described in more detail by meansof examples. The purpose of the examples is not to restrict the scope ofthe claims.

EXAMPLES

Materials and Apparatuses

Ultrasonic device: Hielscher UP400S (from company Hielscher)

Beads mill equipment: Bühler PML2 (from company Bühler GmbH, Germany)

Particle size and zeta potential measurement tool: Malvern ZetasizerNanoZS.

Zeta negative carboxylated nanodiamonds: uDiamond® Molto Vox nanodiamondpowder (commercially available from company Carbodeon). Said commercialzeta negative carboxylated nanodiamond powder acid value was determinedto 34.7, as a service from Ketek Oy, Finland.

The nanodiamond acid value was determined by titration. Every samplemeasurement was conducted twice, with a sample size of 1.5 g pertitration. The titrations were carried out with automated Metrohmtitrator. Determination of acid functions: Solid samples were weightedaccurately (1.5 g). The samples were dispersed into 75 ml of neutralizedethanol (water content 0.5 wt. %), using Hielscher 400 W ultrasonicunit. The prepared samples were titrated with 0.1 M KOH (in methanol),using phenoliftalene as an indicator. The samples were treatedcontinuously with Argon gas flow during the titration. The titration endpoint was detected by using an indicator and with potentiometric means,using Methrohm Solvotrode electrode and drawing a titration curve.

The EDX analysis (Zeiss Ultra Plus Gemine) revealed the appliednanodiamond material is essentially free of nitrogen contained surfacefunctions. The applied acceleration voltage was 20 kV.

N-Methyl-2-pyrrolidone (NMP), Assay ≥99.5% was purchased from VWRChemicals/Prolabo, Assay ≥99.5%.

1-Ethyl-2-pyrrolidone (NEP), Assay ≥98% was purchased from AppliChemPanreac.

The product zeta potential values were measured of samples diluted to0.1 wt-%. The product particle size distributions were measured ofsamples diluted to 0.5 wt-%.

The solvent based nanodiamond dispersion moisture contents weredetermined by Karl Fischer method at Ketek Oy, Finland.

Reference Example—Beads Milling of Zeta Negative CarboxylatedNanodiamond Suspension without pH Adjustment Prior the Milling

Test 1

Deionized water (300 g) and uDiamond Molto Vox zeta negativecarboxylated nanodiamond powder (25 g) having zeta potential −47 mV weremixed together using a magnetic stirrer. As a pre-treatment the mixturewas subjected to ultrasonic treatment in a vessel for 30 min with 100%intensity in order to reduce the size of nanodiamond agglomerates. Theultrasonic treatment was carried out without any mechanical mixing. Agreyish, 7.69 wt-% nanodiamond suspension was formed, and a substantialfraction of nanodiamond material was subprecipitated to the vesselbottom right after ending the ultra-sonic treatment.

The resulting nanodiamond suspension was then subjected to beadsmilling. Already at the initial milling phase, the mill pressure levelsstarted to rise, indicating of a possibility of clogging of the beadsmill sieve system. In order to reduce the inside pressure, the mill wasrun without any pumping for 5 minutes time and the sieve was cleaned. Asthe milling was continued, the pressure started to rise again, and themilling had to be stopped after 20 minutes time.

Test 2

Deionized water (345 g) and uDiamond Vox zeta negative carboxylatednanodiamond powder (25 g) having zeta potential −47 mV were mixedtogether using a magnetic stirrer the resulted mixture was subjected toultrasonic treatment as pre-treatment for 30 min under magneticstirring.

The resulting 6.8 wt-% nanodiamond suspension was then subjected tobeads milling. Prior milling itself the mill all the possible oxygen wasremoved away by pumping water through the mill. The nanodiamondsuspension (370 g) was added slowly, within 10 minutes period into thesystem, and the pump speed was adjusted to only 10% (3 kg/h), and thenanodiamond slurry was circulated through an ultrasonic station. Afterthe whole mixture had been added the pump speed was increased to 15% (5kg/h), and the actual processing was commenced.

During beads milling, analytical samples were taken out on regular 10minutes intervals for direct particle size and zeta potentialmeasurement.

During the beads mill processing, the zeta potential rises from negativeto positive values (FIG. 1). This is due to reduction of zeta-negativecarboxylic acid functions into zeta-positive hydroxyl groups.

Also particle size distribution of the carboxylated nanodiamondsuspension changes during the beads milling. Initially, a decrease inparticle size distribution could be detected, but start to rise again asthe processing was continued (FIG. 2). This is due to agglomerationcaused by formation of zeta positive hydroxyl functions.

The agglomeration can be seen in FIG. 3 which shows nanodiamondprecipitation in a hose of the beads mill apparatus. FIG. 4 showsclogging of sieve system of the beads mill apparatus.

The clogging occurs because reduction of carboxylic acid functions intohydroxyl groups takes place. As hydroxyl groups carry positive zetapotential agglomeration takes place.

Example (According to the Present Invention)—Beads Milling of pHAdjusted Zeta Negative Carboxylated Nanodiamond Suspension

1000 ml of deionized water was mixed with 0.25 ml of 28% ammonia inwater in order to adjust pH of the resulting ammonia suspension intoalkaline region. The received suspension initial pH was measured to10.3. 180 g of prepared ammonia suspension was introduced into the millfor pre-wetting the beads and to fill the hoses of beads mill apparatus.

525 g of deionized water and 45 g of zeta negative carboxylated uDiamondMolto Vox nanodiamond powder were mixed with a conventional magneticstirrer to form a nanodiamond suspension. The suspension initial pH was3.7. The suspension pH was adjusted by adding 1.5 ml of 28% aqueousammonia, followed by ultrasonic treatment for 30 minutes, using H14blade and 40% amplitude. The suspension was stirred with conventionalmagnetic stirrer (100 rpm), with simultaneous cooling with an ice-bath.The resulting 7.9 wt-% nanodiamond suspension pH was 9.0.

Said suspension was then subjected to beads milling, using 30 micronzirconia beads. The milling was initialized with 10% pump speed (3kg/h), for a 20 minutes period, circulating the suspensionsimultaneously through an ultrasonic station. After 20 minutesprocessing time, the pump speed was increased to 15% (5 kg/h), withcontinued ultrasonic treatment. The milling was continued until 90minutes total processing time was reached.

The resulting zeta negative single digit carboxylated nanodiamonddispersion (marked as sample KT-1010-12) final pH was 8.5 and the zetapotential was measured to −45.2 mV (FIG. 5). The particle sizedistribution was the following: D10 1.63 nm; D50 2.89 nm; and D90 6.61nm. FIG. 6 shows particle size distribution of carboxylated nanodiamonddispersion after beads milling.

Example (According to the Present Invention)—Highly Zeta Negative SingleDigit Aqueous Nanodiamond Dispersion Zeta Potential Stability andTunability within pH Range of 2 to 13

In Table 1 are disclosed measured zeta potentials of single digitcarboxylated nanodiamond dispersion at pH range from 2 to 13. pH of thedispersion was controlled with either HCl or NaH₄OH and treated withsonication for 30 min. The zeta potentials were measured from diluted0.1 wt-% nanodiamond samples. The zeta negative single digitcarboxylated nanodiamond dispersion is stable when the zeta potential is−37.5 mV or over −37.5 mV. By fine-tuning the nanodiamond dispersion pHinto a region of around 10 to 11, it is possible to reach zetapotentialvalues exceeding −70 mV, also at this very low nanodiamondconcentration. At higher nanodiamond concentrations, the results will befurther elevated.

TABLE 1 pH zeta potential (mV) 2.2 13.9 3.0 0.8 4.2 −28.7 5.1 −37.9 6.8−46.3 7.0 −53.7 7.4 −54.8 8.8 −61.9 9.8 −70.3 10.9 −70.7 12.0 −62.1 12.4−48.4

The dispersion stability of pH adjusted carboxylated nanodiamonddispersion samples within pH range of 2 to 13 is featured in FIG. 7.

Example (According to the Present Invention)—Preparation of Highly ZetaNegative 2.2 wt. % Single Digit Carboxylated Nanodiamond Dispersion inNMP

The evaporator water bath was preheated up to 45° C. 190 g of 5 wt. %aqueous highly zeta negative, carboxylic acid functionalized nanodiamonddispersion was weighted into 2 liter round bottomed flask, followed byaddition of 490 g of NMP solvent. The resulting mixture was evaporatedin the following sequences: 1000 mbar down to 200 mbar (in threeminutes), down to 100 mbar during additional one minute evaporation,down to 50 mbar in still another minute of evaporation, and down to 15mbar by 10 minute total evaporation time. Said steps were carried outwithout placing the flask into evaporator water bath. Thereafter,evaporation was continued by placing the flask into water bath (T=45°C.), rising the water bath temperature simultaneously into 80° C. during25 minutes. The evaporation was continued for 8 minutes at 80° C. Theresulting dispersion was weighted to 466.4 g. Resulting dispersion watercontent was measured to 0.49 wt. % by Karl Fischer titration. The ovendried sample revealed that produced NMP based nanodiamond dispersionnanodiamond concentration is 2.2 wt. %

The dispersion exhibited the following particle size distribution: D10:3.37 nm; D50: 6.13 nm; D90: 14.1 nm. The dispersion zeta potential wasmeasured to −48.3 mV.

Example (According to the Present Invention)—Preparation of Highly ZetaNegative 1.1 wt. % Single Digit Carboxylated Nanodiamond Dispersion inNEP

The evaporator water bath was preheated up to 45° C. 200 g of 5 wt. %aqueous highly zeta negative, carboxylic acid functionalized nanodiamonddispersion was weighted into 4 liter round bottomed flask, followed byaddition of 200 g of ion exchanged water and 990 g of NMP solvent. Theresulting mixture was stirrer vigorously for 5 minutes time. Then, themixture was evaporated in the following sequences: 1000 mbar down to 200mbar (in three minutes), down to 100 mbar during additional one minuteevaporation, down to 50 mbar in still another minute of evaporation, andto less than 20 mbar by 18 minute total evaporation time. Said stepswere carried out without placing the flask into evaporator water bath.Thereafter, evaporation was continued by placing the flask into waterbath (T=45° C.), rising the water bath temperature during 10 minutes to65° C. Then, the evaporation was continued until reaching the totalevaporation time of 50 minutes.

The resulting dispersion was weighted to 932.5 g. Resulting dispersionwater content was measured to 0.19 wt. % by Karl Fischer titration. Theoven dried sample revealed that produced NMP based nanodiamonddispersion nanodiamond concentration is 1.1 wt. %

The dispersion exhibited the following particle size distribution: D10:2.35 nm; D50: 3.57 nm; D90: 6.75 nm. The dispersion zeta potential wasmeasured to −63.8 mV.

The invention claimed is:
 1. A method for producing an aqueous zetanegative single digit carboxylated nanodiamond dispersion, the methodcomprising adjusting pH of an aqueous zeta negative carboxylatednanodiamond suspension to at least 7, and subjecting the pH adjustedaqueous suspension to beads milling, wherein zeta potential of theaqueous zeta negative carboxylated nanodiamond suspension prior to thepH adjustment is more negative than −30 mV measured at pH from 3 to 5 ata nanodiamond concentration of at least 0.1 wt %.
 2. The methodaccording to claim 1, wherein the pH is adjusted to between 7 and
 14. 3.The method according to claim 1, wherein the zeta negative carboxylatednanodiamond suspension is pre-treated before the beads milling.
 4. Themethod according to claim 3, wherein the pre-treatment isultrasonication.
 5. The method according to claim 1, wherein the pH isadjusted with a Brönsted or a Lewis base selected from the groupconsisting of ammonium hydroxide, ammonia, NaH₄OH, methyl amine, diethylamine, pyridine, tri-methyl ammonia, lithium hydroxide (LiOH), sodiumhydroxide (NaOH), potassium hydroxide (KOH), barium hydroxide (Ba(OH)₂),caesium hydroxide (CsOH), strontium hydroxide (Sr(OH)₂), calciumhydroxide (Ca(OH)₂), and rubidium hydroxide (RbOH).
 6. The methodaccording to claim 1, wherein the beads milling of the pH adjustedsuspension is assisted by ultrasonication.
 7. The method according toclaim 1, wherein zeta potential of the zeta negative single digitcarboxylated nanodiamond dispersion is more negative than −35 mV,measured at pH higher than
 7. 8. The method according to claim 1,wherein zeta potential of the zeta negative single digit carboxylatednanodiamond dispersion is more negative than −60 mV, measured at pHhigher than 8.5.
 9. The method according to claim 1, whereinconcentration of the zeta negative single digit carboxylated nanodiamondparticles in the dispersion is greater than 1 wt-%.
 10. The methodaccording to claim 1, wherein D90 average primary particle sizedistribution of the zeta negative single digit carboxylated nanodiamondparticles is from 2 nm to 30 nm.